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Chapter 1: Introduction Information Technology Department Bryar Hassan (BSc & MSc Eng.) [email protected] Database Management System (DBMS) • DBMS contains information about a particular enterprise – Collection of interrelated data – Set of programs to access the data – An environment that is both convenient and efficient to use • Database Applications: – – – – – – – Banking: transactions Airlines: reservations, schedules Universities: registration, grades Sales: customers, products, purchases Online retailers: order tracking, customized recommendations Manufacturing: production, inventory, orders, supply chain Human resources: employee records, salaries, tax deductions • Databases can be very large. • Databases touch all aspects of our lives Database System Concepts 1 University Database Example • Application program examples – Add new students, instructors, and courses – Register students for courses, and generate class rosters – Assign grades to students, compute grade point averages (GPA) and generate transcripts • In the early days, database applications were built directly on top of file systems Database System Concepts 2 Drawbacks of Using File Systems • Data redundancy and inconsistency – Multiple file formats, duplication of information in different files • Difficulty in accessing data – Need to write a new program to carry out each new task • Data isolation — multiple files and formats • Integrity problems – Integrity constraints (e.g., account balance > 0) become “buried” in program code rather than being stated explicitly – Hard to add new constraints or change existing ones • Atomicity of updates – Failures may leave database in an inconsistent state with partial updates carried out – Example: Transfer of funds from one account to another should either complete or not happen at all Database System Concepts 3 Drawbacks of Using File Systems (Cont.) • Concurrent access by multiple users – Concurrent access needed for performance – Uncontrolled concurrent accesses can lead to inconsistencies • Example: Two people reading a balance (say 100) and updating it by withdrawing money (say 50 each) at the same time • Security problems – Hard to provide user access to some, but not all, data Database System Concepts 4 Levels of Abstraction • Physical level: describes how a record (e.g., customer) is stored. • Logical level: describes data stored in database, and the relationships among the data. type instructor = record ID : string; name : string; dept_name : string; salary : integer; end; • View level: application programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes. Database System Concepts 5 View of Data • An architecture for a database system Database System Concepts 6 Data Models • A collection of tools for describing – – – – • • • • • Data Data relationships Data semantics Data constraints Relational model Entity-Relationship data model (mainly for database design) Object-based data models (Object-oriented and Object-relational) Semi-structured data model (XML) Other older models: – Network model – Hierarchical model Database System Concepts 7 Relational Model • Relational model • Example of tabular data in the relational model Database System Concepts 8 A Sample Relational Database Database System Concepts 9 Data Manipulation Language • Language for accessing and manipulating the data organized by the appropriate data model – DML also known as query language • Two classes of languages – Procedural – user specifies what data is required and how to get those data – Declarative (nonprocedural) – user specifies what data is required without specifying how to get those data • Structured Query Language (SQL) is the most widely used query language Database System Concepts 10 Database Design • The process of designing the general structure of the database: • Logical Design – Deciding on the database schema. Database design requires that we find a “good” collection of relation schemas. • Business decision – What attributes should we record in the database? • Computer Science decision – What relation schemas should we have and how should the attributes be distributed among the various relation schemas? • Physical Design – Deciding on the physical layout of the database Database System Concepts 11 Database Design? • Is there any problem with this design? Database System Concepts 12 Database Design Approaches • Normalization Theory – Formalize what designs are bad, and test for them • Entity Relationship Model – Models an enterprise as a collection of entities and relationships • Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects – Described by a set of attributes • Relationship: an association among several entities – Represented diagrammatically by an entity-relationship diagram Database System Concepts 13 The Entity-Relationship Model • Models an enterprise as a collection of entities and relationships – Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects • Described by a set of attributes – Relationship: an association among several entities • Represented diagrammatically by an entity-relationship diagram What happened to dept_name of instructor and student? Database System Concepts 14 Storage Management • Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system. • The storage manager is responsible to the following tasks: – Interaction with the file manager – Efficient storing, retrieving and updating of data • Issues: – Storage access – File organization – Indexing and hashing Database System Concepts 15 Transaction Management • What if the system fails? • What if more than one user is concurrently updating the same data? • A transaction is a collection of operations that performs a single logical function in a database application • Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g., power failures and operating system crashes) and transaction failures. • Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database. Database System Concepts 16 Database Users and Administrators Database System Concepts 17 Database Architecture • The architecture of a database systems is greatly influenced by the underlying computer system on which the database is running: – – – – Centralized Client-server Parallel (multi-processor) Distributed Database System Concepts 18 Recap • • • • • • • • • • • Database Concepts and DBMS Using File Systems vs Database Systems Levels of Abstraction Data Models and The E-R Model Data Manipulation Language Database Design Database Design Approaches Storage Management Transaction Management Database Users and Administrators Database Architecture Database System Concepts 19